Precipitation is the sole input of regional water resources in mountainous or hilly areas that are not traversed by large rivers. A prerequisite for using isotopic techniques to study the regional water cycle of a mountainous area is to examine the stable isotopic composition of its precipitation. The findings are of great significance for in-depth understanding of the water-cycle processes. In this study, each event of precipitation was sampled and used to investigate the characteristics of stable hydrogen and oxygen isotopes (δ2H and δ18O, respectively) in precipitation on the Jiaolai Plain and its surrounding areas. NCEP/NCAR data was used for the wind speed and direction, relative humidity, and precipitable amount in the study area during the sampling period. The water vapor sources of precipitation over the plain were revealed through a comparative analysis of seasonal variations in precipitation isotopes, between the Global Network of Isotopes in Precipitation (GNIP) stations located along different vapor transport paths. The results showed that the local meteoric water line (LMWL) was δ2H = 6.38 δ18O + 0.72, with a gradient less than 8. This indicated that the precipitation process was affected by non-equilibrium evaporation occurred when the drops fell below the cloud base. Significant temperature and amount effects existed in the δ18O of precipitation, although the altitude effect was not significant. The water vapor source of the precipitation was controlled predominantly by the East Asian Monsoon from June to September, with the main source being evaporation from the adjacent Pacific Ocean. The plain was controlled by Westerlies from October through May, with the predominant vapor source being local evaporation. Water vapor from the polar region had minimal impact. During the sampling period, water vapor brought by Typhoon Lekima produced heavy precipitation on the plain. There was a significant depletion of δ18O in the precipitation at that time, indicating the existence of the cloud–rain zonal effect. These findings can serve as the basis for studying surface water–groundwater–seawater transformation.

The integrated use of isotopic and hydrochemical tracers is an effective approach for investigating complex hydrological processes of groundwater. Thorough understanding of recharge and quality of the groundwater is usually a prerequisite for effective groundwater management. This study investigated the water level, stable isotope and hydrochemistry of groundwater around the Qinghai Lake to reveal the recharge sources, hydrochemical evolution and water quality of groundwater. The relative altitudes of groundwater level ranged from -1.27 to 122.91 m with hydraulic gradient ranging from -6.20 to 43.14‰relative to the water level of Qinghai Lake, indicating most of the groundwater was flowing into the lake. Most of the groundwater points lay close to the local meteoric water line, and the slope of Local Evaporation Line of groundwater (LEL: δ2H = 6.08 δ18O-3.01) was lower than the slopes of the LMWL, indicating that the groundwater were recharged primarily from precipitation at different altitude in the basin, though it had undergone varying degrees of evaporation. The hydrochemical analysis showed that the groundwater was mainly freshwater and its hydrochemical type was Ca-Mg-HCO3. The groundwater chemistry was mainly controlled by carbonate dissolution around Qinghai Lake. Furthermore, high TDS and high concentrations of Na+, Mg2+, Cl-, or SO42- in several groundwater were caused by the recharge source of lake water, the recharge source of fissure water, or by the dissolution of evaporite. The main sources of nitrate (NO3-) in groundwater around Qinghai Lake were animal feces and sewage, suggesting that the pollution of groundwater should be paid more attention in animal husbandry areas on the Qinghai-Tibet Plateau, although the industrial and urbanization rates were relative low on the plateau. The scientific planning and engineering management of livestock manure and wastewater discharge in animal husbandry regions are very necessary to be carried out urgently, which could not only protect water resources for drinking, but also contribute to human health and sustainable development of the ecological environment of the Qinghai-Tibet Plateau.